[RTTY] bartg sprint 75 sv1dpi

[Kok Chen]

On Sep 18, 2011, at 9:51 PM, Kostas Stamatis wrote:

> I noticed that in 75 baud is more difficult to print low signals. At 
> least there were 2-3 times i couldn't print guys who i was hearing them.

Assuming that filters are optimized for both the 45.45 baud and the 75 baud cases, a transmitter that uses 75 baud requires about 2 dB more signal power to reach the same character error rate as a transmitter that uses 45.45 baud.  For example, needing to increase transmitter power from 100 watts to 165 watts.

A 2 dB change is definitely noticeable for RTTY during quiet conditions, where 2 dB reduction in transmitter power could make the error rate increase from 3% to 15%.

IMHO, when there is deep QSB (selective fades or flat fades), a 2 dB change is pretty much a wash between 45.45 baud and 75 baud.

If the modem that is optimized for 45.45 baud does not change the internal filter for 75 baud operation, the SNR does not go up.  However, something even worse happens since a 75 baud signal will cause some pretty severe inter-symbol interference (a signal QRM'ing itself, so to speak :-).  You can even see this effect when using 45.45 baud, if you use a crystal filter in a rig that is too narrow, without also having an appropriate shape for binary 45.45 baud data.

Additionally, for digital modems (not just software modems, but also things like the DXP38 and 599zx that uses DSP) one extra thing comes into play.  Depending upon the implementation, if the number of sampling points per data bit is not increased for 75 baud operation, it causes a larger sampling jitter, and that too can cause more decoding errors, even under QSB conditions.

73
Chen, W7AY